The present invention relates to pigment dispersing additives for use in pigment dispersions of coating compositions.
Pigments used to impart colors to coating compositions are typically added in the form of dispersions. It is important to stabilize these dispersions so that the pigment particles remain suspended in the dispersion medium rather than settling. When settling occurs, the pigments can form a solid flocculated mass that is often difficult to resuspend.
Often, it is desirable to use metallic or metallic-effect pigments. These so-called "flake" pigments are comprised of particles that are denser or larger than other pigment particles; as such, they tend to settle more quickly. Coating compositions containing metallic and metallic-effect pigments are commonly used in automotive original equipment manufacture and refinish applications. They are usually spray-applied at a relatively low viscosity to obtain a glossy, lustrous appearance with a smooth surface and good distinctness of image. They are usually also supplied at relatively low viscosities, to avoid the need for extensive adjustment of viscosity prior to use.
Typically, settling is more severe and occurs more quickly as viscosity drops. Thus, settling of pigments is a common problem when metallic pigment dispersions are added to low-viscosity spray-applied coating compositions.
The settling properties of a pigment dispersion or a coating composition to which the pigment dispersion is added can be modified by using certain pigment dispersing additives that work by modifying the rheology of the coating composition. Typically, these pigment dispersing additives increase low-shear viscosity by producing a secondary structure. For example, one class of pigment dispersing additives works by forming hydrogen bonds that are easily disrupted under high-shear conditions, such as when a coating composition is spray-applied. Thus, anti-settling properties are improved without sacrificing application properties such as flow, leveling, or metallic flake orientation.
Another class of pigment dispersing additives, the "associative" thickeners, work by forming hydrophobic micelles in an aqueous environment. Similarly, inorganic particulate materials such as magnesium silicate clays, hectorite clays or fumed silicas work by forming hydrogen bonds in either an aqueous or a non-aqueous environment. Pigment dispersing additives comprising organic polymeric materials such as waxes, castor oil derivatives, polyurethanes, or carboxylated acrylics that work by a hydrogen-bonding mechanism have also been described, and these can be made suitable for use in either an aqueous or a non-aqueous environment. For a pertinent review, see "Pigment Stabilization Through Mixed Associative Thickener Interaction," by David J. Lundberg and J. Edward Glass, Journal of Coatings Technology, Vol. 64 No. 807, April, 1992.
Finally, U.S. Pat. No. 5,168,105 describes a three-package coating composition comprising, in part, a non-aqueous metallic pigment package that includes a theological additive. The rheological additive disclosed is an acid functional acrylic polymer prepared by conventional vinyl addition polymerization yielding a random structure.
Rheological additives which exhibit good wetting properties as well as good stability when mixed with pigments such as aluminum flake or titanium dioxide are, therefore, desirable.